Conventional vortex shedding flowmeters include a bluff body or shedder placed in a fluid for generating alternating vortices downstream of the shedder, a phenomenon known as a Von Karman vortex street. The frequency and amplitude of these vortices are typically detected and measured by pressure sensors in order to determine the mean flow velocity. The frequency of the generated vortices is linearly proportional to the mean flow velocity, and the amplitude of the pressure pulse is proportional to the square of the flow velocity times the fluid density.
Sensors used to detect the vortices often include diaphragms which fluctuate in response to alternating pressure variations generated by the vortices. For example, in U.S. Pat. No. 3,948,098 to Richardson et al., pressure applied to the diaphragms is transferred to a piezoelectric bimorph device sealed within a sensor housing via an electrically non-conductive hydraulic fill fluid. This type of sensor, however, cannot be used for measuring the flow velocity of extreme temperature fluids, because of the temperature limitations of the fill fluid, and the piezoelectric bimorph.
Meters used for measuring high temperature fluids, such as steam, are constructed without fill fluids. These meters include a shedder in the fluid flow path which oscillates or vibrates as vortices are "shed" from opposite sides thereof. These vibrations induce bending moments which are detected by a pair of sensors hermetically sealed in the vortex shedder. An example of this type of meter is shown and described in U.S. Pat. No. 4,437,350 to Hisashi Tamura, et al.
Since these devices rely on the detection of vibrations of the shedder, the sensors are made to be highly sensitive for detecting slight oscillations. This high sensitivity coupled with the very large mass of the shedder (approaching 500 grams in larger meters), however, makes them adversely susceptible to mechanical vibrations transmitted through the pipe and the shedder. To eliminate this noise, the sensors in the patent to Tamura above consists of two piezoelectric sensing elements which are both electrically divided with respect to the neutral axis of the shedder for symmetrically dividing the oscillations into tensile and compression stress components. In essence, the sensing elements act as if there were four independent sensing elements. Complicated circuitry is then required to compare the signals from each piezoelectric element for cancelling out the vibration and common mode noise components of the signal.